A thrust bearing such as one employed in a submersible pump or a submersible motor is usually subjected to a condition in which a thrust load imposed thereon increases when the motor or pump is put into operation. The value of the thrust may reach more than 1,000 kgf even in a small sized pump and, thus, such a thrust bearing is one of the portions to which great attention needs to be paid during the manufacture of these pumps.
A thrust bearing of a tilting pad type which involves precisely machined divided elements has previously been used in submersible equipment. In order to allow such a bearing to be used with an adequate lubricating effect, highly viscous lubricating liquid is required and, therefore, it has been necessary to encase and seal lubricating oil or the like within the bearing.
Accordingly, in the conventional thrust bearing of this type, there have been problems related to the sealing and cooling of the lubricating liquid. In addition, other problems concern the deterioration of the lubricating liquid, its low loading capacity and its low degree of reliability. Also, it cannot be used for hot liquids such as hot spring water or underground water at a temperature of 200.degree. C. or more.
Spiral groove bearings have been known which are free from the drawbacks referred to above in connection with the thrust bearing of the tilting pad type.
Spiral groove bearings are described, for example, in U.S. Pat. No. 3,497,273 of E. A. Muijderman et al. E. A. Muijderman has played and continues to play a leading international role in the research on and development of spiral groove bearings.
In fact, the fundamental shape of the spiral groove bearing and the theoretical analysis thereof were established by Muijderman and his research group and announced not only by Muijderman et al. but also in a number of publications.
The shape of the spiral groove bearing has already been established, as well as the theory relating thereto. According to that theory, when, for example, a lubricating oil is used as an operating fluid in a spherical spiral groove bearing that is 100 mm in diameter, the bearing can withstand a load of about 100,000 kgf even when the bearing is rotated at a speed as low as 200 r.p.m. (see: Nikkei Mechanical, p 78-83, May 28, 1979).
However, the kind of equipment which currently utilizes such spiral groove bearings in fact includes VTRs, Video Disks, Magnetic Disks, Record Players, and Pumps having a low load capacity. All of these devices have a load capacity far lower than the extremely high load suggested in the above-mentioned theory.
Thus, although the spiral groove bearing has been theoretically established as having a high load capacity, the actual bearing capacity (load capacity) is far from that which is suggested by the theory, and no reasons have been given to explain why there is such a gap between theory and practice, or how it is possible to make theory coincide with practice in this respect.
Various devices have been proposed for improving the capacity and reliability of bearings. These improvements mainly concern: (a) reductions in starting resistance, (b) the shape of the groove, (c) countermeasures against the wear that occurs during starting and stopping, (d) reductions in end play in the axial direction and (e) methods of machining the groove. Item (e) is the main problem and it has thus far caused much concern. Methods such as etching, spark erosion, pressing and plating have been proposed, among which etching has been the one most recommended. Most of the proposals that utilized the etching method were concentrated in the period from 1973 to 1980.
During the development of such a technical trend, the following items have of late become important problems: (i) how to make a spiral groove bearing which has excellent antiwear capacity and which keeps its initial shape; (ii) how to improve the load carrying capacity of a spiral groove bearing; (iii) how to make a bearing which has sufficient load carrying capacity when lubricated with specific liquids (for example, water, sea water, corrosive liquids, high-temperature liquids fluids mixed with foreign matter, etc.); and (iv) how to obtain an accurately controlled groove shape.
Against this background, Japanese Laid-Open Patent No. 57-15121 of Tanaka et al. was proposed in 1982. Tanaka discloses solutions to several problems, namely how to control the shape of a groove, how to make the bearing element resistant to wear, and how to form a groove which satisfies the requirements for good shape. In the light of the development of bearings up to the date of his invention Tanaka starts from the standpoint of a known metal bearing, and proceeds by adding a ceramic coating as a way to increase the resistance to wear. In addition, Tanaka realizes that the thickness of this ceramic coating can serve as a means for accurately controlling the depth of the groove, and discloses the fact that this makes possible the use of shot-blasting, which essentially strips the ceramic coating from unmasked areas to form grooves in the coating which then serve as the grooves for the bearing.
Tanaka makes a statement that a bearing produced by the method disclosed displays a capacity close to its theoretical value, but this is because the Tanaka bearing is only used under a small load. When the load is increased, the Tanaka bearing will withstand neither the load itself nor repeated starting and stopping, both characteristics being essential in bearings of this type. In addition, such a coated bearing is not only more difficult to make, but is in fact more costly. In the first place, a ceramic coating cannot be applied directly to a smooth metallic surface. There are various techniques for applying a ceramic coating to the surface of a metallic material, but even where the metallic material surface is roughened, it is not possible to obtain a high degree of bonding strength. Further, the known methods of depositing the ceramic material, such as plasma deposition, result in a ceramic which is relatively porous.
Moreover, it has also been found that shot-blasting, which is believed to be the only practical way to roughen the surface in order to cause adherence of the ceramic, deforms the metallic disk so that the central portion of the surface which is shot-blasted is raised and the edges are depressed, i.e. in cross section the disk is slightly bowed. For this reason, it is difficult to make the thickness of the ceramic layer deposited thereon both flat on its outer surface and uniform in thickness. The tendency is for the ceramic layer to have a thin area at the center of the disk and a much thicker area around the edges after the free surface thereof has been lapped to make it smooth. This greatly reduces the ability to make the depth of any groove which is blasted in such a layer uniform in depth. Moreover, it is not always easy to accurately lap the ceramic layer such as to produce the desired thickness.
Further, the cost of the steps necessary to form the ceramic layer on the metallic surface together with the shotblasting and finishing steps cause the bearing to be costly.